The present invention is related to cryostat construction and in particular to means for supporting a thin-walled horizontal penetration tube so as to permit relative motion between inner and outer cryostat vessels without unduly stressing the penetration tube. The present invention is also related to the construction of cryostats for containing coolants such as liquid helium used to cool the superconductive windings of a main magnet for a medical diagnostic nuclear magnetic resonance (NMR) imaging system.
Conventional cryostats for NMR imaging systems typically require disruption of the cryostat vacuum for the purpose of inserting temporary stiffening supports to protect the magnet and internal components during transportation. Transportation of such superconducting magnets is therefore seen to require re-establishment of internal vacuum conditions after the magnet is disassembled to remove the temporary support. This is a time-consuming operation. In conventional cryostat designs, large elastomer seals are commonly employed to facilitate assembly and disassembly. Furthermore, other cryostat designs have included a non-metallic cryostat bore tube wall to prevent eddy current field distortions when NMR gradient coils are energized. These gradient coils are typically disposed within the bore of the magnet assembly. However, both elastomer seals and non-metallic bore tubes are permeable to gases and either design results in contamination of the internal vacuum conditions during long term operation of the device. Therefore, periodic pumping of the cryostat is required. Moreover, seal replacement requires periodic total shutdown and warming of the superconductor windings to ambient temperature conditions. Accordingly, it is seen that it is desirable to permanently maintain vacuum conditions within a cryostat, not only for purposes of transport, but also for purposes of long term operation.
Conventional cryostat designs also typically employ an access port for addition of coolants such as liquid helium in awkward positions on top of the cylindrical cryostat structure. Such coolant access means are conventionally disposed on the curved side surface of the cryostat and adds significantly to the overall dimensions of the cryostat assembly. This is a significant disadvantage for cryostats employed to house superconducting windings which are used to produce a high intensity magnetic field for whole body NMR imaging application. Since the bore tube of the magnet assembly must be sized to accommodate the human form, with the bore tube typically being approximately one meter in diameter, the overall size of the magnet and cryostat significantly affects the cost, most notably of the magnet itself but also the cost of the room or structure in which it is housed. Accordingly, it is desired to provide a cryostat housing having horizontal access means for addition of liquid coolant and for penetration of electrical leads, these means being located at the end surface of the cryostat.
In order to minimize thermal conduction between inner and outer cryostat vessel walls, a thin walled penetration tube is employed. Additionally, vacuum conditions are maintained between inner and outer cryostat vessels and a system of supporting ties is employed at each end of the inner vessel so as to support the inner vessel within the outer vessel. It is also noted that in these cryostat structures, intermediate vessels and thermal radiation shields are also typically present to increase the effectiveness of the cryostat. However, a system of supporting ties preferably permits axial motion so that the inner vessel may be moved axially and locked into a fixed position during transport. This permits transport of the magnet and cryostat assembly in a fully charged condition, that is, in a condition in which the magnet and coil superconductors have already been cooled to below their critical temperatures. This permits rapid system installation.
However, relative motion of the inner and outer cryostat vessels can severely stress the thin walled penetration tube. Accordingly, one of the objects of the present invention is to provide a support for this relatively delicate penetration tube. In addition to axial motion which is deliberately employed for purposes of transport to lock the inner cryostat vessel in a fixed position, transverse and rocking motions of the inner vessel can also occur. Thin walled penetration tubes fixed to both the inner and outer cryostat vessels could be subject to potentially damaging mechanical stresses as a result of the relative motion between the inner and outer cryostat vessels. Additionally, differential thermal expansion and contraction effects can also operate to induce stress in penetration tubes which are firmly anchored to both the inner and outer vessel walls. The large transverse displacements and rocking motions of the inner vessel during shipment could subject the thin walled penetration tube to high bending stresses beyond yield strength. Additionally, this tube must also withstand axial loads as a result of the cryostat vacuum and magnetic interactions with external ferromagnetic objects.